# Does frequency shifting double needed bandwidth? [duplicate]

Suppose we have a low pass signal and it's spectrum is like this:

If we define bandwidth as the difference between signal minimum and maximum frequencies, above picture says that the signal has a bandwidth of $B$.

If we shift this signal to frequency $f_s$ by multiplying it with $cos(f_st)$, we get this spectrum:

So now signal bandwidth is : $f_s+B-(f_s-B)=2B$ . Is it correct to say this signal shifting doubled signal bandwidth ?

Suppose we want to transmit analog human voice over $f_s$ frequency. Before shifting, needed bandwidth was 4 KHz, but after shifting we need 8 KHz of bandwidth, is it true?

• heh, it's really just that question, with much nicer graphics – Marcus Müller Jan 12 '17 at 14:30